We watched a concrete pour where concrete was revibrated after it had set up. The vibrator head did not "readily penetrate" the concrete as described in ACI 309. In fact the inspector said "it took 15 to 30 seconds of vibrating at each pass for the probe to enter the older concrete." The "older" concrete had been placed two hours earlier.

What happened in the concrete as it was revibrated and would it have been better to leave the concrete as it was without revibrating and place a cold joint?

Revibrating previously consolidated concrete momentarily liquefies the concrete again. Formation of calcium hydroxide is the primary chemical process that occurs during the first two hours after concrete is placed. The other major product of hydration is calcium silicate hydrate, which gives the concrete its hardness and durability. Formation of calcium silicate hydrate begins in earnest only after several hours have elapsed.

The concrete reaches initial set somewhere in that process. After initial set, formation of the more brittle, weaker calcium hydroxide continues but falls behind the calcium silicate hydrate formation, which accelerates dramatically between initial and final sets.

When revibration occurs after the initial set, it breaks down some of the calcium hydroxide that has already been formed. That allows freshly placed concrete next to the revibrated concrete to join with it, rather than introducing a construction joint, and it again becomes a monolithic concrete structure.